1. Field of the Invention
The present invention relates to an image scanning device that is used in an image forming apparatus for reading images, the image forming apparatus including, for example, copiers, printers, facsimiles and combined machines thereof. More particularly, the invention relates to an image scanning device wherein light from a light source is guided into a light guide and converted into a linear light beam which is applied onto a reading position. In a case where the intensity of light emitted from the light source varies due to change in the characteristics of the light source or the like, the image scanning device of the invention can quickly detect the change in light intensity and provide quick and proper correction of the intensity or distribution of the linear light beam emitted from the light source through the light guide.
2. Description of the Related Art
The image forming apparatus, such as copiers, printers, facsimiles and combined machines thereof, employs the image scanning device for digitizing or copying a document image and the like.
Such image scanning devices typically employ a contact image sensor (CIS system) or a charge-coupled device (CCD system).
The image scanning device employing the CIS system can be downsized so as to be suitable for reading document. However, this type of scanning device has a short focal depth and is not adapted to scan a three-dimensional object. On the other hand, the image scanning device employing the CCD system is adapted to scan the three-dimensional object but requires an increased optical path length between the reading position irradiated with light and an image reading element. As a result, this type of image scanning device has a larger size than that employing the CIS system.
In the image scanning device employing the CCD system, a document 2 placed on a glass platen 1 is typically scanned as follows. As shown in FIG. 1, light from a light source 3a disposed in a first slider unit 3 driven by a motor (not shown) or the like and the light emitted from the light source 3a and reflected by a reflector plate 3b are applied onto a reading position P for the document 2. The light reflected from the document 2 and carrying image information is reflected by a first reflective mirror 3c disposed in the first slider unit 3.
The light carrying the image information and reflected by the first reflective mirror 3c is reflected by a second reflective mirror 4a and a third reflective mirror 4b disposed in a second slider unit 4 driven by a motor (not shown) or the like. The reflected light is focused by a lens 5 onto a photoreceptive surface of an image reading element 6.
Subsequently, the above-described first slider unit 3 and second slider unit 4 are operated to scan the whole document 2 on the glass platen 1.
While the conventional scanning devices have generally employed fluorescent lamps, halogen lamps and the like as the above light source 3a, the use of light-emitting diode (LED) light source or organic electroluminescent element is drawing attention from the standpoint of power saving and low heat generation.
More recently, devices, as disclosed in Japanese Unexamined Patent Publications No. 2007-300536 and 2009-25679, have been proposed wherein an LED light source is provided at an end of an elongate light guide so that the light from the LED light source is applied by the light guide in the form of the linear light beam.
In the case where the light guide emits the light from the LED light source in the form of the linear light beam, it is necessary to make proper settings of intensity and distribution of the linear light beam applied from the LED light source through the light guide.
The proper settings of intensity and distribution of the linear light beam applied from the LED light source through the light guide may be made as follows. The above first slider unit and second slider unit are moved to locate the light guide with the LED light source at a suitable position relative to the glass platen, or commonly at position corresponding to a shading sheet as a white reference member provided at an end of the glass platen.
The linear light beam is applied onto the shading sheet via the light guide while the reflected light beam therefrom is made incident on the photoreceptive surface of the image reading element. Proper adjustments are accomplished by correcting the intensity of the linear light beam applied from the LED light source through the light guide and an amplification factor of the image reading element.
If the intensity of the light emitted from this LED light source varies due to the change in the characteristics of the LED light source, the intensity and distribution of the linear light beam applied from the LED light source through the light guide is corrected properly. Each time the correction is made, the above-described operations must be done which include: moving the first slider unit and second slider unit to locate the light guide with the LED light source at position corresponding to the shading sheet; applying the linear light beam to the shading sheet through the light guide and making the reflected light therefrom incident on the photoreceptive surface of the image reading element; and correcting the intensity of the linear light beam applied from the LED light source through the light guide and the amplification factor of the image reading element.
However, such operations are cumbersome and time consuming so that the image forming apparatus is significantly decreased in image formation efficiency. Particularly in the device disclosed in Japanese Unexamined Patent Publication No. 2009-25679 wherein the light guide is provided with the LED light sources at opposite ends thereof and the LED light sources are activated in turn to make adjustments, the operations become more complicated and time consuming. An image scanning device equipped with an automatic document feeder for automatically feeding a large quantity of documents will cause a significant decrease in the image formation efficiency of the image forming apparatus.